1. Field of the Invention
The invention relates to a process interface of a process gas analyzer operating by a transmitted light method, comprising a measuring head, which contains an optoelectronic element emitting or receiving light and is mounted on a process flange of a plant part containing or carrying a gas to be analyzed, a purging tube, which extends through the process flange between the optoelectronic element and the interior of the plant part, a window, closing off the purging tube at the end opposite from the optoelectronic element and separating the optoelectronic element from the interior of the plant part, a purging gas feed, entering the purging tube in a region close to the window, and an annular part, arranged in the interior of the purging tube opposite the entrance of the purging gas feed and coaxial in relation to the purging tube.
2. Description of the Related Art
EP 2 428 793 A1 discloses a conventional process interface.
In the case of gas analyzers operating by a transmitted light method, the light of a light source is passed through the gas to be analyzed and subsequently detected. In the case of a laser spectrometer, for example, the light is generated wavelength-selectively and is detected in a broadband range. As a difference from this, in the case of a non-dispersive infrared (NDIR) gas analyzer, for example, the light is generated in a broadband range and is detected wavelength-selectively. In the case of in-situ process gas analyzers, the light source and the detector are accommodated in different measuring heads, which are mounted on process flanges on diametrically opposite sides of a plant part containing or carrying the process gas to be measured (for example, an exhaust gas line, or vessel, flue). In order that the light source and the detector do not come into contact with the process gas, which is often aggressive, hot and contains dust, they are arranged behind windows. The window closes off one end of a purging tube, which at its other, open end enters the gas-carrying plant part and is flushed with a purging gas. The purging gas is chosen such that its spectral absorption lines lie outside the absorption lines of the process gas that are used for the measurement. The purging gas leaves from the open ends of the opposing purging tubes, so that the length of the measuring path for the absorption measurement of the process gas is determined by the distance between the open ends of the two purging tubes.
The greater the throughflow of purging gas, the more effectively it succeeds in keeping the windows free from contaminants from the process gas. However, a high consumption of purging gas entails correspondingly high costs and influences the measurement on account of the great quantities of purging gas that get into the measuring path between the opposing purging tubes.
The conventional process interface disclosed in the aforementioned EP 2 428 793 A1 has a purging gas feeding unit with a tubular flange entered by a purging gas feed. Pushed into the flange is a purging ring, which contains a system of interconnected grooves, in order to distribute the purging gas uniformly around the circumference, and finally conduct it via bores into the interior space of the tube. The bores are aligned such that they conduct the respective stream of gas away from the window to be purged. This is intended to ensure a homogeneous, virtually swirl-free inflow, so that the required quantity of purging gas can be reduced. Since the window is not flushed directly, when steam is used as the purging gas it may condense on the window.
DE 1 993 225 U describes a purging air attachment for the protection of optical surfaces, in the case of which an annular part arranged in the interior of the purging tube opposite the entrance of the purging gas feed and at a distance from the window forms between itself and the purging tube an annular gap that is open in the direction of the window and closed in the direction of the interior of the plant part. The purging gas is conducted completely to the window and deflected there, so that the stream of purging gas is subsequently in a swirled state. Turbulences disturb the measurement, in particular the measurement with a laser, and require a higher consumption of purging gas to achieve the same purging effect in comparison with an undisturbed homogeneous purging gas flow.
It is known from DE 10 2004 018 534 B4 to conduct the purging gas into the purging tube through a porous wall. Here, too, turbulences of the purging gas flow through the purging tube occur.
A process interface with a long purging tube is likewise known from EP 2 169 385 A1.